Power transformers (such as transformers having primary voltages of several hundreds of kilovolts (kV) and delivering powers lying in the range a few megavolt amps (MVA) to several hundred MVA) are extremely expensive pieces of equipment in systems for interconnecting electricity transmission networks or “grids”. It is therefore very useful to be able to keep such transformers in service for as long as possible, since a transformer breakdown or fault can have major economic consequences due to the distribution network being interrupted.
In addition, faults ouch as short circuits can run the risk of explosion or fire.
It is therefore very important to be able to determine the presence of a fault associated with a transformer winding.
A known solution that problem consists in using FRA. That technique consists in measuring the impedance of a transformer winding over a wide range of frequencies and in comparing the result of the measurement with a set of reference measurements. In order to measure impedance as a function of frequency, it is possible to perform frequency scanning using a sinewave signal.
Thus, FIG. 1 is a theoretical diagram of a circuit 1 for performing frequency analysis on an impedance corresponding to the impedance of a transformer winding that is to be measured.
The circuit 1 comprises:
a network analyzer 2;
three same-value test impedances Z1; and
an impedance ZT corresponding to the impedance to be measured of a transformer winding.
The network analyzer 2 generates a measurement signal S. The measurement signal S is a frequency-scanned sinewave signal. By way of example, the impedances Z1 are the impedances of measurement cables and they generally have a value that is equal to 50 ohms (Ω). R is the signal measured between the first end of ZT and ground. T is the signal measured between the second end of ZT and ground. The analyzer 2 then determines voltage gain k as a function of frequency, as defined by the following relationship:
  k  =      20    ⁢                  log        10            ⁡              (                  T          R                )            
The gain k contains the information needed for studying the impedance ZT and is equal to:
  k  =      20    ⁢                  log        10            ⁡              (                  Z1                      Z1            +            ZT                          )            
When the impedance Z1 is equal to 50 Ω, this gives:
  k  =      20    ⁢                  log        10            ⁡              (                  50                      50            +            ZT                          )            
Impedance is measured over a very wide range of frequencies that can extend from a few hertz (Hz) to about 10 megahertz (MHz).
The same measurement must be performed on a reference winding. The reference winding can either be another phase which is assumed to have no fault, or the same winding as measured previously when it had no fault, or the winding of an identical transformer. This measurement likewise produces a gain k′ as a function of frequency and corresponding to the reference winding.
A first solution then consists in examining by eye any differences between the curves representing k and k′ as a function of frequency. That solution nevertheless presents certain problems.
An examination by eye performed by an expert can be highly subjective and can lack transparency.
A second solution consists in calculating statistical indicators suitable for revealing the differences between the two curves. Such statistical indicators can be constituted, for example, by correlation coefficients calculated over different frequency ranges.
Nevertheless, the use of such statistical indicators also gives rise to certain problems.
Thus, certain faults cannot be identified; this applies for example to the magnetic circuit of the transformer being grounded or to circulating current causing the winding to heat.
Similarly, such use of statistical indicators can lead to certain faults being confused; for example poor grounding of the transformer tank can be confused with damage to a winding.